The present invention relates to internal combustion engines and, more particularly, to automatic compression release mechanisms employed in internal combustion engines.
Automatic compression release mechanisms are employed in internal combustion engines to provide for improved engine performance at a variety of engine speeds. Such mechanisms typically include a component, actuated based upon engine speed, that varies an exterior surface characteristic of a cam lobe along which a push rod governing an exhaust valve of the engine rides. Specifically, when engine speeds are low, such as during the starting of the engine, a protrusion is created on the cam lobe such that the exhaust valve tends to open slightly during the compression stroke of the engine, which facilitates the starting of the engine. However, when engine speeds are higher, such as during normal operation of the engine, the protrusion is eliminated such that the exhaust valve remains closed during the compression stroke of the engine to maximize engine power.
Automatic compression release mechanisms of this type often employ a weight that is rotatably affixed to a portion of the camshaft such as a cam gear. As the rotational speed of the camshaft increases, centrifugal forces acting on the weight tend to cause the weight to rotate outwards (away from the camshaft axis). However, the weight is typically biased by a spring towards the camshaft so that, while the engine is at low speeds, the weight is rotated inward toward the camshaft. Because the movement of the weight is dependent upon the rotational speed of the camshaft, the movement of the weight can be used to govern components associated with the cam lobe to produce the desired speed-dependent variation in cam lobe shape. Commonly these components include a shaft having a recessed side and an unrecessed side, which is mounted along the exterior surface of the cam lobe. When the weight is rotated inwards, the unrecessed side of the shaft extends outward beyond the exterior surface of the cam lobe producing a protrusion, and when the weight is rotated outwards, the recessed side of the shaft faces outward and the protrusion on the cam lobe is largely or entirely eliminated.
In many engines, it is desirable to employ an automatic compression release mechanism having as few components as possible, in order to simplify and consequently reduce the costs of the mechanism. This can be achieved to some extent by integrally forming as a single piece the weight and the shaft having the recessed and unrecessed sides, such that rotation of the weight directly causes rotation of the shaft. For similar cost-related reasons, it often is desirable for engines to employ simply-formed and inexpensive components throughout the cam shaft assembly. For example, the cam gear can be molded out of plastic or diecast as a single piece. Also, the cam lobe can be integrally formed as part of the cam gear, or at least fixedly attached onto, the cam gear.
However, the desire for simplified cam shaft assembly components can conflict with the desire for simplified automatic compression release mechanisms having fewer components. In particular, given close proximity of the cam gear and cam lobe, the weight and shaft of the automatic compression release mechanism cannot be effectively mounted on the side of the cam gear facing the cam lobe. At the same time, if the weight and shaft are mounted on the other side of the cam gear opposite the cam lobe, the shaft must then extend through the cam gear and onto the cam lobe to provide the desired operation. Retention of the weight and shaft on the cam gear then becomes problematic. In particular, clasps or other simple components that could be attached at the end of the shaft to keep the shaft in place relative to the cam gear cannot effectively be employed unless the shaft extends beyond the cam lobe, which renders the shaft excessively long and fragile and increases manufacturing costs (particularly where it is desired to manufacture the shaft using powdered metal technologies).
It therefore would be desirable if a new automatic compression release mechanism were developed that employed few and inexpensive components and was capable of being implemented on simple camshaft components such as an integrally-formed cam gear and cam lobe. It further would be desirable if the new automatic compression release mechanism employed an integrally-formed weight and shaft that was small and inexpensive to manufacture, and at the same time was easily mounted on and retained with respect to the cam gear.
The present inventors have discovered a simplified automatic compression release mechanism that can be implemented on a camshaft having a cam gear and cam lobe attached together (or integrally formed), and that requires few, inexpensive parts, is robust and is easy to assemble. The mechanism includes an arm having an integrally formed weight and shaft. The arm is mounted on the cam gear by inserting the shaft into a tube extending through the cam gear so that the shaft extends past the gear and along the surface of the adjacent cam lobe. The weight is then locked into place in the axial direction (along an axis of the tube) by way of a retaining mechanism existing on the side of the cam gear on which the weight is located. In one embodiment, the retaining mechanism includes a pillar extending outward from (and formed integrally with) the cam gear, and a retaining disk that is fitted onto the pillar. A lip of the retaining disk extends over the weight and thereby retains the weight and shaft in position with respect to the cam gear. Consequently, it is not necessary that the shaft of the arm be excessively long to extend beyond the cam lobe in order for the shaft and weight to be retained.
In particular, the present invention relates an automatic compression release mechanism for implementation in an internal combustion engine including a cam shaft assembly having a cam gear, a cam lobe positioned along a first side of the cam gear, the cam lobe including a notch, a hollow tube passing from the first side of the cam gear to a second side of the cam gear and substantially aligned with the notch, and a support extending from the second side of the cam gear proximate the hollow tube. The automatic compression release mechanism further includes an arm including a weight and a shaft, where a first end of the shaft is coupled to a near end of the weight and a second end of the shaft includes a recessed portion, where the shaft is rotatably positioned within the hollow tube so that the weight is positioned along the second side of the cam gear and the second end of the shaft protrudes out of the hollow tube beyond the first side of the cam gear and into the notch. The automatic compression release mechanism additionally includes a retaining member positioned onto the support so that the weight is positioned in between the retaining member and the hollow tube and retained with respect to the cam gear.
The present invention further relates to an automatic compression release mechanism including a cam lobe, a cam gear having a first side and a second side, the cam lobe abutting the first side, and an arm including a weighted portion positioned proximate the second side of the cam gear and a shaft coupled to the weighted portion and extending through a tube from the second side of the cam gear to and beyond the first side of the cam gear and into a notch within the cam lobe. The automatic compression release mechanism additionally includes means for retaining the arm in a substantially constant position with respect to an axis of the tube.
The present invention additionally relates to a method of assembling an automatic compression release mechanism on an internal combustion engine. The method includes providing a camshaft assembly including a cam lobe and a cam gear having a first side and a second side, where the first side of the cam gear is adjacent to the cam lobe, where the cam lobe includes a notch along its exterior surface, where the cam gear includes a hollow tube that extends through the cam gear and is aligned with the notch along the first side of the cam gear, and where the cam gear further includes a pillar protruding from the second side. The method further includes providing an arm having a weight with a first side and a second side and a shaft having a first end and a second end, where the first end of the shaft is attached to the weight, and where the second end of the shaft includes a recessed portion. The method also includes inserting the shaft of the arm through the hollow tube so that the second end of the shaft including the recessed portion is positioned at least partly within the notch, and so that the first side of the weight is proximate the hollow tube. The method additionally includes coupling a retaining member to the pillar so that at least a portion of the retaining member extends over the second side of the weight and prevents excessive movement of the shaft out of the hollow tube and excessive movement of the weight away from the second side of the cam gear.